What it shows
Water with a little salt conducts a tiny current. Each probe that touches the water passes enough base current to switch a BC547 on, so one LED lights for each level reached.
The science
A common probe at the bottom is connected to 9 V. Each of the three level probes feeds a base through a 10 kΩ resistor. When the water touches a level probe, a small current flows through the water and the resistor into the base. The BC547 amplifies this about 200 times, so the LED in its collector lights. Salt lowers the water resistance so the base current is enough even with tens of kilohms in the water.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A tall plastic bottle with the top cut off
- Three BC547 transistors
- Three 10 kΩ resistors and three 470 Ω resistors
- Green, yellow and red LEDs
- Five stainless steel nails or screws as probes with wires
- A 9 V battery with snap connector
- A breadboard, a pinch of salt and tape
Safety
For this project: Battery power only, never a mains supply anywhere near the water. Keep the breadboard dry and away from splashes. Wash hands after handling the salt water and probes.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Tape the common probe to the inside of the bottle with its tip near the bottom and connect it to the 9 V rail.
Tape three level probes at about 3 cm, 6 cm and 9 cm heights, tips pointing into the bottle.
Place the three BC547 transistors on the breadboard with all emitters on the ground rail.
Join each level probe to the base of its transistor through a 10 kΩ resistor.
Join a 470 Ω resistor from the 9 V rail to each LED long leg, and each LED short leg to the transistor collector.
Pour in water with half a teaspoon of salt dissolved, until it just covers the first probe.
Watch the LEDs light one by one as the water rises past each probe; empty slowly and watch them go off.
Test with plain tap water too and note if the response is slower or the LED dimmer.
What you should see
The green, yellow and red LEDs come on in turn as the water rises and go off in reverse order as it falls.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why add salt to the water?
The salt provides ions that carry current, lowering the water resistance.
2What is the transistor doing?
It amplifies the tiny probe current into a large collector current for the LED.
3Why is a 10 kΩ resistor in each base line?
It limits base current so the transistor and probe are not stressed.
4What would happen to the steel probes over weeks?
They corrode because of the current in salt water, so replace or rinse them.
Take it further
Add a fourth transistor stage that sounds a buzzer when the top probe is reached, as an overflow warning.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each